1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and in particular to removal of residue in forming a hole of high aspect ratio.
2. Description of Related Art
With the recent demand for high integration, downsizing of semiconductor devices is more and more accelerated. Accordingly, there has also been a demand for reducing the dimensions such as of contact holes. However, an interlayer insulating film forming a contact hole is required to have a certain degree of thickness because of the necessity of mitigating parasitic resistance, for example. As a result, contact holes have been required to be etched in such a way that a high aspect ratio (in particular, an aspect ratio of 10 or more) can be imparted thereto.
In the case of a silicon oxide film, which is a generally used interlayer insulating film, dry etching is performed using a fluorocarbon gas containing a small ratio of fluorine relative to carbon in a molecule, in order to form a contact hole having a high aspect ratio.
In performing such etching using a fluorocarbon-based gas under the condition of raising etching selection ratio for photoresist, a polymer film that contains carbon and fluorine as main components may be deposited on the bottom of the contact hole. It is known that such a polymer film can be removed using oxygen plasma.
Japanese Patent Laid-Open No. 11-145111 describes that contact resistance is recognized as being increased when such a polymer film is removed using oxygen plasma. In order to resolve this problem, a method has been suggested, which successively carries out a plasma etching process using a fluorocarbon-based gas and an oxygen plasma treatment process. In particular, this method includes applying bias voltage to the substrate during the plasma etching process using a fluorocarbon-based gas, and causing oxygen plasma without applying bias voltage to the substrate when the plasma etching process is switched to the oxygen plasma treatment process, thereby removing fluorine from the reaction chamber.
On the other hand, it is known that, in manufacturing a high-aspect hole, light etching is performed to remove damaged layers and etching residue (so-called “deposits”) after opening a hole. For example, Japanese Patent Laid-Open No. 09-162172 discloses a method for removing damaged portions formed in a base silicon layer. Specifically, in forming a contact hole in an interlayer insulating film, the method includes introducing into a chamber a process gas comprising CF4+O2 with a CF4 gas content of 60 to 80% by volume with a predetermined introduction pressure and flow rate, and applying high-frequency power to the introduced gas to have plasma produced for removal of the damaged portions. In this way, a side-etching amount of a contact hole is considered to be minimized by specifying the introduction ratio, introduction pressure and flow rate of CF4 gas.
Use of a fluorocarbon-based gas, such as CF4 gas, in performing such light etching may partially remove deposits, however, may not completely remove the deposits. In addition, use of a fluorocarbon-based gas in performing light etching produces reaction products including carbon (C) and fluorine (F) with C—F bond (polytetrafluoroethylene-based material: (—CF2CF2—)n).
Being different from such light etching, Japanese Patent Laid-Open No. 11-345874 describes that, in etching an oxide film using a resist mask, carbon atoms in the resist undergo reaction with fluorine radicals in the process gas to permit adhesion of a similar polytetrafluoroethylene-based material onto the sidewall of the hole. This reference further describes that, for the removal of the adherents, O2 plasma treatment is combined with a chemical washing process.
In order to raise the removal rate of the residual deposits after light etching, wet processing using a wash solution may be performed. However, the reaction products with C—F bond that have been produced after light etching are hydrophobic. Accordingly, adhesion of such reaction products in the hole may resultantly block the entry of a wet washing solution during post treatment. This tendency is particularly prominent in the holes having a high aspect ratio.
FIGS. 1(a) to 1(c) are process cross-sectional views illustrating procedures of wet washing after light etching, and of burying a contact plug after the washing. These figures illustrate a case where a contact is formed between a bit line and upper wiring.
As shown in FIG. 1(a), after light etching, contact hole 3 is formed in interlayer insulating film 2 on bit line 1, and residual deposits 4 and reaction products 5 with C—F bond are adhered to the bottom of contact hole 3. If wet processing is performed in this state using wash solution 6, such as a solution of dilute hydrofluoric acid (DHF), wash solution 6 will not reach the bottom of the contact hole, as shown in FIG. 1(b), because reaction products 5 are hydrophobic, thereby disabling removal of deposits 4.
In this way, in the state where washing has not been sufficiently performed, a conductive material is buried, with deposits 4 being left in hole 3. This may resultantly raise a problem of high resistance by which the characteristics of the semiconductor device will be deteriorated.